c - 错误 : ‘asm’ undeclared (first use in this function)

Question

我在编译过程中遇到以下错误:

error: ‘asm’ undeclared (first use in this function)
 EXCHANGE( s, *(a) );
 ^

在调用宏的头文件中，如下所示:

EXCHANGE( s, *(a) );

宏的实际定义如下:

#define EXCHANGE(R,M) asm volatile ( "xchg   %1, %0" : "+m" (M), "+r" (R) )

宏调用和定义存在于同一个头文件中。出了什么问题？

我正在使用CMAKE构建项目，CFLAGS如下:

set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Werror")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wextra")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-missing-braces")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-missing-field-initializers")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wformat=2")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wswitch-default")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wcast-align")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wpointer-arith")
#-Wno-deprecated-declarations to suppress the deprecation errors with newer version of JSON-C 
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-deprecated-declarations")
#set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wbad-function-cast")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wstrict-overflow=5")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Winline")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wundef")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wnested-externs")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-cast-qual")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wshadow")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wunreachable-code")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wfloat-equal")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wstrict-aliasing=2")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wredundant-decls")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wold-style-definition")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-unused-parameter")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99")

#set(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS} -O0 -g3 -ggdb")
set(CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS} -O3")
set(CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS} -E")

Answer 1

要么使用 __asm__ 而不是 asm 和 -std=c99，或者使用 -std=gnu99

来自 GCC 文档 https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/Extended-Asm.html

The asm keyword is a GNU extension. When writing code that can be compiled with -ansi and the various -std options, use __asm__ instead of asm (see Alternate Keywords).

和https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/Alternate-Keywords.html#Alternate-Keywords :

-ansi and the various -std options disable certain keywords. This causes trouble when you want to use GNU C extensions, or a general-purpose header file that should be usable by all programs, including ISO C programs. The keywords asm, typeof and inline are not available in programs compiled with -ansi or -std (although inline can be used in a program compiled with -std=c99 or -std=c11). The ISO C99 keyword restrict is only available when -std=gnu99 (which will eventually be the default) or -std=c99 (or the equivalent -std=iso9899:1999), or an option for a later standard version, is used.

The way to solve these problems is to put __ at the beginning and end of each problematical keyword. For example, use __asm__ instead of asm, and __inline__ instead of inline.

Other C compilers won’t accept these alternative keywords; if you want to compile with another compiler, you can define the alternate keywords as macros to replace them with the customary keywords. It looks like this:

#ifndef __GNUC__
#define __asm__ asm
#endif

-pedantic and other options cause warnings for many GNU C extensions. You can prevent such warnings within one expression by writing __extension__ before the expression. __extension__ has no effect aside from this.

-std=gnu99 启用 GNU 扩展，例如 asm，同时仍保持类 C99 语言。

C99 标准

GCC 这样工作以符合 C99 标准，该标准将双下划线符号标记为保留。

来自C99 N1256 standard draft 7.1.3 “保留标识符”1:

Each header declares or defines all identifiers listed in its associated subclause, andoptionally declares or defines identifiers listed in its associated future library directionssubclause and identifiers which are always reserved either for any use or for use as filescope identifiers.

• All identifiers that begin with an underscore and either an uppercase letter or anotherunderscore are always reserved for any use.

否则，如果 asm 是预定义的，则合法的程序如下:

int asm = 0;

会变成非法的。

测试程序

#include <assert.h>
#include <stdint.h>

int main(void) {
    uint32_t io = 0;
    __asm__ volatile (
        "movl %0, %%eax;"
        "inc %%eax;"
        "movl %%eax, %0;"
        : "+m" (io)
        :
        : "%eax"
    );
    assert(io == 1);
}

在 Ubuntu 17.10、GCC 7.2 上测试。